Various medical conditions are addressed by fitting an eye with an intraocular lens to replace a natural crystalline lens of the eye. Such medical conditions include aging effects, or can result from accidents or from exposure to atypical environmental conditions.
For example, development of a cataract is a common condition experienced with age. The eye is typically fitted with an intraocular lens during cataract surgery. A goal of cataract surgery has long been to provide, postoperatively, unaided (without wearing glasses) high-quality distance, intermediate, and near vision.
For many years, basic attempts to restore vision have included surgically emptying a capsular bag in which the natural crystalline lens of the eye resides and refilling it with an accommodating polymer that matches the behavior of a juvenile lens. While such attempts have received considerable attention an effective actualization remains elusive today in part because properties of homogeneous polymers are insufficient to mimic properties of an inhomogeneous natural crystalline lens. In an article entitled “Accommodating IOLs: Emerging Concepts and Designs” published July 2004 in Cataract & Refractive Surgery Today, Samuel Masket MD describes difficulties in characterizing a crystalline lens in situ, which is subject to forces exerted by adjoining tissues, leading to an inability to create an implant having desired properties under forces exerted by adjoining tissues postoperatively.
Emptying the capsular bag may induce some damage to tissues other than the crystalline lens. The healing process includes transient and permanent changes in adjoining tissues. Postoperative changes in adjoining tissues vary with the nature of the implant material and its contact with an anterior capsule of the eye. Masket further appraises of a need to control postoperative reactions of lens epithelial cells in order to reduce the tendency for fibrometaplasia, typically observed for anterior subcapsular lens epithelial cells after implant surgery. As well, any crystalline lens characterization is necessarily performed on an imperfect lens slated for invasive medical removal with the desire of providing a perfect intraocular prosthesis postoperatively. Even if a characterization of the crystalline lens from an earlier age would have been available, the surrounding tissues also vary with age rendering such characterization insufficient.
Implanting a fixed focus (monofocal) lens has been attempted in the prior art with limited degree of success. Postoperatively the combination of the remaining adjoining tissues and fixed focus lens provide a limited degree of accommodation (controlled focus variability) compared to the juvenile natural lens. It has been found that such monofocal prosthesis combinations may provide between 0.5 to 1.5 diopter pseudoaccommodation after surgery. In comparison, research by Scheiman, Mitchell and Wick, Bruce in Clinical Management of Binocular Vision, Lippincott, N.Y., 1994 suggests that on average a juvenile lens provides 18 diopters variability in average amplitude of accommodation. The average amplitude of accommodation at a given age may be estimated by Hofstetter's formula: 18.5 minus one third of the patient's age in years. Therefore, while such an intraocular implant may provide clearer vision post cataract operation, the limited degree of post operative accommodation requires additional visual aids such as glasses or contact lenses.
Musket provides a survey of intraocular lens devices characterized by at least one movable lens optic within the capsular bag. A change in the position of a single optic providing focus variability, although simple in design, is considered to be insufficient to provide accommodation over a wide diopter range. In addition, it is recognized that the flexibility of the capsular bag remains an important performance aspect for such implant designs. Single optic flexible prostheses which fill the entire capsular bag and remain stationary while changing an anterior/posterior dimension to vary optical power subject to forces provided by the ciliary body are also considered insufficient. Some attempts suffer from material incompatibilities while others remain theoretical. Dual optic prostheses have been implanted however suffer from low optical power variability in the range of 2.5 diopters.
Tunable Liquid Crystal (TLC) optical devices are described, for example, in commonly assigned International Patent Application WO/2007/098602, which claims priority from U.S. 60/778,380 filed on Mar. 3, 2006, both of which are incorporated herein by reference. TLC optical devices are flat multi-layered structures having a Liquid Crystal (LC) layer. The liquid crystal layer has a variable refractive index which changes in response to an electric field applied thereto. Applying a non-uniform (spatially modulated) electric field to such liquid crystal layer, provides a liquid crystal layer with a non-uniform (spatially modulated) index of refraction. Moreover, liquid crystal refractive index variability is responsive to a time variable electric field. In general, TLC's are said to have an index of refraction which varies as a function of an applied drive signal producing the electric field.
With an appropriate geometry, a variety of optical components employing TLC optical devices can be built, for example: a tunable lens, a corrective optical element, an optical shutter, iris, etc. Tunable Liquid Crystal Lenses (TLCLs) provide significant advantages being thin and compact. The performance of TLC lenses may be measured by a multitude of parameters, including: a tunable focus range, optical power (diopter) range, power consumption, transmittance, etc. The optical power of a TLC lens refers to the amount of ray bending that the TLC lens imparts to incident light (and more specifically to an incident light field representative of a scene) passing therethrough.
Recently tunable liquid crystal lenses have been proposed for use in active accommodation. For example:
A notable prior art experimental attempt at providing a TLC lens is Naumov et al., “Liquid-Crystal Adaptive Lenses With Modal Control” Optics Letters, Vol. 23, No. 13, p. 992, Jul. 1, 1998, which describes a one hole-patterned layered structure defined by a non-conductive center area of an electrode covered by a transparent high resistivity layer. With reference to FIG. 1, TLC 100 includes: top 102 and bottom 104 substrates, and a middle Liquid Crystal (LC) layer 110 sandwiched between top 112 and bottom 114 liquid crystal orienting layers. LC orienting layers 112/114 include polyimide coatings rubbed in a predetermined direction to align LC molecules in a ground state, namely in the absence of any controlling electric field. The predetermined orientation angle of LC molecules in the ground state is referred to herein as the pre-tilt angle. The average orientation of long liquid crystal molecular axes in a liquid crystal layer is referred to as a director. An electric field is applied to the LC layer 110 using a uniform bottom transparent conductive electrode layer 124 of Indium Tin Oxide (ITO), and the top hole-patterned conductive ring electrode layer 122 of Aluminum (Al). The low resistivity hole-patterned conductive layer 122 together with the high resistivity layer 126 immediately below the hole-patterned conductive layer 122 form an electric field shaping control layer 128. In accordance with Naumov's approach, the reactive impedance of the LC layer 110 which has capacitance and the complex impedance of the high resistivity layer 126 play a strong role, requiring driving the TLCL via specific voltage and frequency parameter pairs to minimize root means square deviation from a parabolic phase retardation profile for corresponding desired optical power settings (transfer function).
Unfortunately, from a manufacturing perspective it is very difficult to produce, with useful consistency, the required sheet resistance of high resistivity material having high optical transparency for the highly resistive layer 126. It happens that, for millimeter size lenses, the value of R_s, for almost all known solid state materials, is in the middle of an electrical conductivity transition (percolation) zone, where the sheet resistance has a very drastic natural variation with layer 126 geometry. Therefore in practice it is very difficult to produce such TLCLs consistently. Different TLCL's of the same manufacturing batch have slightly different resistances. Such sheet resistance variability coupled with the fact that control is very dependent on the precise LC cell thickness, leads to each such individual TLC lens requiring separate calibration and drive. Two identical such TLC lenses must be used together, with cross-oriented directors, to act on unpolarized natural light.
Despite these drawbacks, in an article published on 7 Apr. 2003 in Optics Express, Vol. 11, No. 7, pp. 810-817 entitled “On the possibility of intraocular adaptive optics”, Naumov et al. presents a theoretical treatise considering the technical possibility of an adaptive contact lens and adaptive eye lens implant using the modal liquid crystal lens described above as a modal liquid-crystal wavefront corrector aimed to correct accommodation loss of the human eye. However, a breadboard demonstrator described, having a 5 mm optical (ring electrode) aperture, provided only some accommodation improvement of about 3 diopters. While amplitude and spectral composition of an applied unipolar AC voltage is theorized for controlling both optical power and radial aberrations of the modal lens, reduction to practice is difficult in view of the specific voltage and frequency parameter pairs required for driving the TLCL to minimize root means square deviation from the parabolic phase retardation profile. Naumov also theorizes control of azimuthal optical aberration components being realized by splitting the annular control ring (122) into sectors with independent control signal components applied to each sector. However, experiments performed by Naumov in providing wireless control have shown that the modal liquid-crystal wavefront corrector cannot develop the required voltage amplitudes across the liquid crystal layer using inductive control and that capacitive control results in rather large voltages being developed in the order of 10V while providing only a limited optical power range of 3 diopters. These results are understood as a direct consequence of the reactive impedance of the LC layer 110 which has capacitance and the complex impedance of the high resistivity layer 126 which play a strong role favoring capacitive wireless control. It remains unclear how capacitive control may be used for actively driving a segmented annular ring electrode to control azimuthal optical aberration components because of complex capacitive interactions between capacitive drive and inter segment capacitances. Photoelectric control while mentioned, is dismissed by Naumov due to a large 1 mW optical source required to shine substantially into the eye during operation. Moreover, at page 814 lines 3 to 4, Naumov et al. expressly state “[their] belie[f that] no wires can be used in the human eye and no battery can be embedded into the lens [prosthesis].”
Another prior art attempt is described by Azar in U.S. Pat. No. 6,638,304 published 28 Oct. 2003 entitled “Vision Prosthesis”. Azar builds on fixed focus optic implants described hereinabove and concludes that multifocal lens implants are less than desirable introducing aberrations due to an inability to select a desired optical axis as typically possible with bifocal glasses because the implanted lens does not permit relative motion between the pupil and the implanted corrective lens. In contrast to Naumov, Azar does not present experimental results based on any actualized tunable liquid crystal lens. Miniaturization of eye glasses sized liquid crystal lenses relied upon by Azar to the size of the capsular bag typically 9 mm by 4 mm has been found impractical for the following reasons: Some embodiments described by Azar (at column 5 line 59 to column 6 line 6) include multitudes of individually addressable electrodes for example arranged concentrically or in a grid pattern to provide spatial modulation together with a liquid crystal layer. Miniaturization of such embodiments is extremely complicated because the numerous traces required to drive each electrode, would introduce electric field components which would impede consorted operation of liquid crystal molecules in the liquid crystal layer to provide a lens of miniature dimensions. In other embodiments (presented in column 6 lines 11 to 17) liquid crystal molecular reorientation is described to employ a magnetic field generated by a current carrying coil. However, power requirements of such a coil are impractical for intraocular prostheses. Other embodiments (presented at column 5 lines 45 to 58) employ electric field drive achieved via differential DC voltages maintained across multiple electrodes arrayed over a liquid crystal layer. Practical application of DC voltage drive is very limited because applied DC voltages have been found to degrade liquid crystal properties. Yet other embodiments (presented at column 6 lines 32 to 39) describe individual addressable lenslet arrays with individual electrodes for each lenslet. However, a very large number of individual lenslets would be required to achieve a smooth refractive index variation across the pupil, correspondingly the inert material separating each lenslet would cover a substantial area of the pupil, as well dispensing the very small amounts of liquid crystal material required for each lenslet during manufacture is limited by surface tension of the liquid crystal material. In contrast, experimental results provided by Naumov, and operational results of tunable liquid crystal lenses presented in commonly assigned International Patent Application WO/2007/098602, which claims priority from U.S. 60/778,380 filed on Mar. 3, 2006, both of which are incorporated herein by reference, show tunable liquid crystal lens reduction to practice using few, typically one spatially non-uniform patterned electrode generating a spatially modulated electric field generated by few, in some embodiments one AC drive signal.
Moreover, the nature of the variability of the index of refraction in response to an applied electric field depends on the physical properties of TLC multi-layered structure, including properties of the liquid crystal layer material, material properties of other layers, geometry, etc. A quasi-linear “functional” relationship between the drive signal applied and the index of refraction of a TLC optical device exists over a usable drive signal variability range. However, the overall relationship is non-linear: In some TLC devices, a physical non-linear effect, known as disclination, is observed as the liquid crystal molecules begin to align with the electric field from a ground state orientation to an orientation dictated by the electric field. In broad terms, when the applied electric field is essentially homogenous, non-linearity means that the change in optical property (e.g. index of refraction) per unit drive signal change varies over the range of optical property change of the optical device. Such disclinations cause optical defects and aberrations in the lens which persist with gradual voltage adjustments necessarily employed in tuning. In “Liquid Crystal Lens with Focus Movable in Focal Plane”, published 2006, in Optics Communications 259, pp. 710-722, Sato specifically points out on page 711 mid page in the left hand column problems with divided electrode structures described by Azar: “The problem in the divided-electrode structure is that disclination lines occur if the potential differences among adjacent subelectrodes become large . . . ”.
With reference to column 6 lines 18 to 26, Azar, based on then known low optical power liquid crystal lenses, addresses optical power insufficiency by employing micromechanical motors, alternatively proposes employing a curved liquid crystal lens having a baseline curvature performing gross correction while local liquid crystal reorientation-induced refractive index variations are employed to fine-tune the gross correction. To date manufacturing LC structures on curved substrates is impractical.
In U.S. Pat. No. 6,576,013 filed 8 Jan. 2002, entitled “Eye prosthesis” Budman et al. describe a cosmetic hard contact lens containing electronics for displaying an iris and pupil image on a color liquid crystal array display device embedded in the hard contact lens. The hard contact lens is not a (functional) replacement for a natural eye lens and is not employed in vision restoration.